Volume change in polypyrrole studied by atomic force microscopy

The electrochemically induced volume change in conjugated polymers is of in terest for actuator applications. Important parameters include the magnitud e and speed of the volume change and how volume correlates with potential a nd current. In this paper, atomic force microscopy was used to examine the expansion and contraction of films of polypyrrole doped with dodecylbenzene sulfonate. The films were photolithographically patterned to allow the scan to start on the substrate, go over the film, and return back to the substr ate, giving an absolute height measurement. The method is described in deta il. This, technique allowed us to map the real-time volume change in-situ i n the out-of-plane direction, which revealed that the film thickness increa sed by over 35% in the reduced state compared to the oxidized state. This c ompares with an inplane strain of similar to2% found in prior studies using bilayers, showing that the volume change is anisotropic. The thickness cha nge was well-correlated with the cyclic voltammograms, and the polymer resp onded with a 3%/s expansion for an applied reducing potential step. The hei ght change depended on film thickness, with films near 1.5 mum undergoing t he greatest strain. The morphologies of the as-grown, oxidized, and reduced states were also examined, but there were no substantial differences. The unexpectedly large out-of-plane strain was confirmed by in-situ mechanical surface profilometry. These measurements also showed that the hardness of t he reduced state was lower than that of the oxidized state.